With energy resources becoming gradually more limited and expensive, it makes an increasingly more attractive business case to produce wind power. Accordingly, during the recent years there has been increasing focus on alternative energy such as wind-generated electricity.
When operating a wind turbine under normal conditions, the blades can usually be pitched to feathered position during stopping of the wind turbine. This can be done in controlled manner in most cases, but during emergency stops, in case of grid drop out, failure in generator or control system, the pitching of the blades in order to achieve the emergency stop can introduce severe uncontrolled loading.
Whilst the turbine's blade pitch control system may be capable of feathering the blades, for example in case of excessive wind velocity, feather systems are provided for emergency blade feathering during failure of the blade pitch control system. A common way to achieve such feathering is to adjust the pitch angle of the blades to approximately 90 degrees, whereupon wind flow over the blades quickly reduces the rotational speed of the rotor. By this manner the torque produced by the blades and hence the rotation of the generator rotor is minimized.
Several attempts have been made to provide wind turbines with systems for feathering the turbine's blades when it is considered necessary to shut the turbine down. In order to reduce operating time at conditions of excessive wind velocity, rapid blade feathering is desirable. On the other hand, feathering at a constant, rapid rate may result in excessive blade stresses due to substantial decelerating torque and reverse thrust developed by the blades as they approach fully feathered positions. Therefore, as the blades are feathered, at the time point where the blade pitch has increased to the point where airflow over the blades no longer develops positive torque on the turbine rotor, the next thing to do would be to reduce the rate of feathering in order to reduce the decelerating torque and reverse thrust developed by the blades, and by this means minimizing blade stresses. For safety reasons, reduction in the pitch rate as a result of the blades approaching their feathered positions should not come at the expense of feathering at a maximum rate while airflow over the blades develops a positive shaft torque.
Emergency pitch systems are typically either hydraulic or electrical.
U.S. Pat. No. 4,462,753 discloses a mechanical/hydraulic pitch system with a separate emergency system for wind turbines. The blade feathering system includes a feather actuator, control means operatively connected thereto and an adjustment means operatively connected to the control means for selectively varying the rate of operation of the feather actuator for feathering the wind turbine blades at a variable rate. One drawback of this invention is a cam follower is required to operate a valve. The cam follower is expensive and troublesome to manufacture. Another drawback of this invention is that it uses a separate actuator to turn the blades during emergency stop and another actuator for normal operation. A further drawback of the disclosure is that the use of an electrically driven pump to turn the blades during emergency. In the case of power failure, this may have fatal consequences.
The emergency pitch problems have previously been solved by introducing a flow control valve in the hydraulic system that limits the pitch speed to about 10 degrees/sec. This pitch speed is a compromise that limits the rotational speed to an acceptable value in most cases. Hereby one can prevent extreme loading of the turbine. The crucial problem is that the flow control valves are inexact. This lack of accuracy involve that flow of 11 degree/sec. may occur. The load can hereby be increased significantly. Another drawback is that the pitch speed depends on the viscosity of the oil and hence the temperature of the oil.
The electrical systems work differently. When the emergency pitch is activated, there is no guarantee for electrical contact in the rest of the turbine. This may be crucial due to the fact that the control system may be out of order. Batteries or other electrical energy sources situated near by the pitch system can bring the blades to feathered position. The drawback with this method is that the generator generally provides a counter torque, however, in the case of electrical dysfunction, the counter torque will no longer be present. Accordingly, this may cause a dramatically increased angular velocity of the rotor, which can activate the speed guide and in the worse case damage components. Furthermore, the aerodynamic loading may introduce severe uncontrolled loading, because the loading, generally speaking, is proportional to the square of the velocity. On the other hand, if the pitch is too fast, the blade will experience a negative lift and, hence, the rotor will pull the turbine forward and introduce extreme load on the blades and tower.
WO 06 007 838 discloses a method of controlling at least one wind turbine blade during the stopping process of the rotor in a wind turbine system. The method optimizes the control velocity of the process in response to one or more feedback values of the system and/or one or more feedback values from the surroundings of the system by altering the angular pitch velocity from 10 degrees/sec during the initial stage of the stopping process to 5 degrees/sec at the final stage of the stopping process. A control system and a wind turbine as well as use hereof are also disclosed. One drawback of this system is the use of a turbine control system during an emergency stop. While it would be advantageous to apply a purely passive mechanical system, the disclosed system uses a turbine control system that requires electrical power with the disadvantageous risk for power failure.